>>A quick way to check this is to decrease the TDR pulse repetition rate
>>and see if the "impedance" changes.
>What would this indicate? Is it the repetition rate I want to decrease or
>the rise time?
My comment was aimed at the case where TDR is used to look at a
track which has a large RC time constant. It now sounds like this isn't
your case, but for what it's worth:
Reducing the repetition rate (typ. 10 kHz or so) of the TDR pulse train may
clarify the measurement results in the case of a large RC time
constant. One typically looks at the rising edge response; usually the
response to the falling edge decays to zero before the next rising
edge is launched. If you set the display timebase to a large value
you can see successive TDR pulses.
If the device under test has a large RC time constant (i.e. the same order as
the TDR rep. rate), the falling edge response won't decay to zero before
the next rising edge is launched. The TDR display will show the superposition
of the rising edge response and the late-time falling edge response.
If the resulting display is interpreted as being solely due to the rising
edge (as it would be in a typical TDR measurement), weird impedances can result.
If I'm not mistaken, this is the effect that Mr. Hill was referring to.
Of course, his comment:
>>>If this explanation goes down in flames I won't be surprised.
goes for me too!